Binary Kuiper Belt Objects

a: semimajor axis; e: eccentricity; i: inclination.
Type:PKBO = Plutino, CKBO = Classical, SKBO = Scattered s: Angular Separation P: Orbital Period [days] DMag: Magnitude Difference between Components
Object a [km] e i [deg] Type Q [arcsec] P [days] DMag
Pluto 19,600 0.00 96 PKBO 0.9 6.4 3.2
1998 WW31 22,300 0.8 42 CKBO 1.2 574 0.4
2001 QT297 ---- --- --- CKBO 0.6 --- 0.5
2001 QW322 --- --- --- CKBO 4.0 --- 0.4
1999 TC36 --- --- --- PKBO 0.4 --- 1.9
1998 SM165 --- --- --- SKBO 0.2 --- 1.9
1997 CQ29 --- --- --- CKBO 0.2 --- 0.3
2000 CF105 --- --- --- CKBO 0.8 --- 0.9
2001 QC298 --- --- --- CKBO 0.17 --- N/A
2003 EL61 49,500+/-400 0.050+/-0.003 234.8+/-0.3 SKBO 1.5 49.12+/-0.03 3.3

Pluto has been known to be binary since 1978. On this basis, many Kuiper Belt researchers openly speculated that other Kuiper Belt Objects might also be found to be binaries. The first compelling example of a binary, after Pluto-Charon, was that of 1998 WW31 (Veillet et al. 2002).

Other observations soon revealed additional binary KBOs (see Table 1). Their orbital elements are poorly determined but should yield to continued observations over the next few years.

By early 2010, about 35 were known out of a total of about 1300 KBOs (i.e. nearly 3%). With strong observational biases against the detection of very close binaries, and against binaries in which the brightness difference between components is large, we can be sure that the binary fraction amongst these objects is much larger, probably exceeding 10%. Binary KBOs are surprisingly common.

Binary Formation

Why is this? The origin of the Kuiper Belt Binaries is a subject of speculation. Straight gravitational capture of one KBO by another is essentially impossible without some process that can dissipate some of the kinetic energy present in the initial motions of the bodies. Collisions provide a natural source of friction, and most speculations about satellite formation involve collisions.

The best known binary is Earth-Moon, which is thought to have formed by an ancient impact that ejected material from proto-Earth into an orbiting disk, from which our satellite quickly accreted. Perhaps Charon was formed by accretion in a disk blown out of Pluto following an ancient impact and the satellites of 2003 EL61 (Haumea) probably formed collisionall, too. However, the smaller binaries listed in Table 1 possess too little mass and gravity for this mechanism to seem plausible. A disk of sufficient mass would not have been formed by any reasonable collision.

Instead, the Kuiper Belt binaries might have formed more directly through low velocity collisions between KBOs. When the relative velocities are smaller than or comparable to the gravitational escape speeds from the colliding objects, some fraction of the collisions will "stick", resulting in peanut shaped contact binaries. Others will bounce, with some ejection of mass (and energy) allowing binaries to form. Still others will collide but not lose enough energy in order to become bound. In this scenario, the binary KBOs are products of low velocity (100 m/s) collisions in the Kuiper Belt.

One problem is that collisions between 100 km sized KBOs are currently very rare: too rare to account for the inferred number of binaries. Independent evidence suggests that the Kuiper Belt was once about 100 times more massive than now, so this suggestion is not completely out of line. But another problem is that the known Kuiper Belt binaries tend to have components of comparable mass. Binaries produced collisionally are most usually highly asymetric in the masses of their components.

Other ideas proposed to account for Kuiper Belt binaries include dynamical friction (the net effect of many small KBOs tugging on nearby objects and causing a net loss of energy and a slow spiralling together). Three-body interactions have also been proposed, in which two objects become more tightly bound when a third carries away some of their kinetic energy. A hybrid mechanism, in which collisions create an unequal binary and then the small component is ejected on the close pass of a third, has also been advanced. The different suggestions make distinct predictions about the ratio of wide to narrow binaries and thus offer a future observational test.

These ideas all require a much denser Kuiper Belt in order to be effective, and thus make the binaries "primordial" features of the Belt. Petit and collaborators have shown that the stability of wide binaries is marginal on timescales comparable to the age of the solar system. They argue that the existing binaries represent just a fraction of those initially present.

Some binaries

1998 WW31

1998 SM165

1999 TC36

David Jewitt.

Kuiper Belt